This invention relates to an ultrasonic wave blood flow meter for measuring the flow rate and the flow speed, etc. of blood by depending upon the Doppler effect of the ultrasonic wave signal reflected from the blood flow.
It is well known that, if an ultrasonic wave is radiated and the Doppler effect of the reflected wave is measured, a signal proportional to the flow rate can be obtained. Furthermore, various methods for obtaining the flow speed from a signal proportional to the flow rate have been proposed. A typical example is the so-called arithmetic method. According to this method, the Doppler signal reflected from the fluid is orthogonally or quadrature-phase detected to obtain two signals V.sub.A and V.sub.B with their phases different by 90.degree. from each other. The signal V.sub.A is differentiated and multiplied with the signal V.sub.B. The d.c. component of this signal is divided by the d.c. component of V.sub.B.sup.2. Then, we obtain a voltage proportional to the flow speed.
However, this method has the following defect. The operation accuracy of the currently available divider is expressed as EQU E=e/V.sub.I,
where V.sub.I is the denominator voltage and e is the standard error. Therefore, if we perform division by using such a divider, the dividing operation error increases in inverse proportion to the voltage V.sub.B. Hence, no accurate flow speed is obtained. When V.sub.B is zero, the output of the divider remains undefined. Due to noise of the input signal, a similar result as in the case of a rapid flow may be obtained. Another defect is that the output in the case without flow or the zero level is not defined well.
This invention is intended to solve the abovementioned problems of the prior art. The objective is to provide an ultrasonic blood flow meter which can keep the operating precision of the divider at a value as constant as possible for such various flow rates and yet can give a zero-frequency output in the absence of flow.